Heating appliances, such as oil-fired furnaces, boilers and water heaters, often have "drum-style" heat exchangers which house a combustion chamber or fire pot. Drum-style heat exchangers are usually cylindrical, however heat exchangers having rectangular or polygonal-prismatic shapes are also known. The combustion chamber is typically composed of a ceramic material and is configured to surround the burner flame to evenly distribute heat within the heat exchanger and to increase the temperature in the combustion zone. A fluid, such as air, is circulated around the heat exchanger for heating and subsequently distributed or stored elsewhere. Cool fluid is returned to the device for heating.
In operation, the combustion chamber is subjected to extreme heat, sometimes in excess of 1500.degree. C., which inevitably causes degradation of the chamber. Therefore it is necessary to provide access to, and permit removal of, the combustion chamber for periodic maintenance, repair and replacement, as required.
Conventional hot-air furnaces having drum-style heat exchangers typically include a rectangular cross-sectioned horizontal duct, extending from a lower housing portion of the heat exchanger to the outside casing of the furnace, through which the combustion chamber may be removed for servicing. Such a design is disclosed in U.S. Pat. No. 2,389,264 to Livar. This design, however, blocks the flow of air around a portion of the heat exchanger, thereby reducing the efficiency of the heat exchanger. As the market prices for fossil fuels continually increase, there is an ever-increasing need for improved efficiency heaters.
The rectangular cross-sectioned horizontal duct of the prior art has further disadvantages. With the advent of side wall-vented furnaces, also known as direct-vented furnaces, the combustion gases within the heat exchanger are pressurized in relation to the air outside the heat exchanger. The presence of a positive pressure inside the heat exchanger necessitates a heat exchanger having a complete pressure seal to prevent combustion gases from leaking into the circulation air and endangering the health and safety of building occupants. There is no similar concern in chimney-vented furnaces, since combustion gases in the heat exchanger are maintained at a negative pressure in relation to the circulation air, and any leaks in the heat exchanger simply result in circulation air being drawn into the heat exchanger. When used in a direct-vented furnace, however, the large opening of the rectangularly-ducted heat exchanger of the prior art is disadvantageous because the rectangular access to the duct is difficult to completely pressure seal. Pressure sealing this prior art heat exchanger design requires careful placement of many fasteners, as well as gasketing, around the perimeter of the opening, which results in increased material and labour costs in manufacture. Accordingly, there is also a need for a heat exchanger facilitating easier means for providing a pressure seal therefor.